1. Field of the Invention
The present invention relates generally to an apparatus and method for a TFCI (Transport Format Combination Indicator) code generator in a CDMA mobile communication system, and in particular, to an apparatus and method for encoding a TFCI in an NB-TDD (Narrowband-Time Division Duplex) mobile communication system.
2. Description of the Related Art
In general, a CDMA mobile communication system (or an IMT-2000 system) transmits data frames of various services such as a voice service, an image service and a data service all together, using a single physical channel. Such service frames are transmitted at either a fixed data rate or a variable data rate. As for the different services transmitted at a fixed data rate, it is not necessary to inform a receiver of a spreading rate of the respective service frames. However, regarding the services transmitted at a variable data rate, it is necessary to inform the receiver of a spreading rate of the respective service frames. In the IMT-2000 system, the data rate is in inverse proportion to the data spreading rate.
When the respective services use different frame transfer rates, a TFCI is used to indicate a combination of the currently transmitted services. The TFCI secures correct reception of the respective services.
FIG. 1 illustrates an example in which an NB-TDD communication system uses the TFCI. Herein, the NB-TDD system employs 8PSK (8-ary Phase Shift Keying) modulation for high-speed transmission, and the TFCI bits are encoded to a code of length 48 before transmission. As shown in FIG. 1, one frame is divided into two sub-frames of a sub-frame#1 and a sub-frame#2. Each sub-frame is comprised of 7 time slots TS#0-TS#6. Among the 7 time slots, the odd-numbered time slots TS#0, TS#2, TS#4 and TS#6 are used for an uplink transmitted from a mobile station to a base station, while the even-numbered time slots TS#1, TS#3 and TS#5 are used for a downlink transmitted from a base station to a mobile station. Each time slot has a structure in which data symbols, a first part of TFCI, a midamble signal, SS symbols, TPC symbols, a second part of TFCI, data symbols and GP are sequentially time-multiplexed.
FIG. 2 illustrates a structure of a transmitter for transmitting a frame in a conventional NB-TDD communication system. Referring to FIG. 2, a TFCI encoder 200 encodes an input TFCI and outputs a TFCI symbols. A first multiplexer (MUX) 210 multiplexes the TFCI symbols output from the TFCI encoder 200 and other signals. Here, the “other signals” refer to the data symbol, the SS symbol and the TCP symbol included in each slot of FIG. 1. That is, the first multiplexer 210 multiplexes the TFCI symbol and the other signals except for the midamble signal of FIG. 1. A channel spreader 220 channel-spreads the output of the first multiplexer 210 by multiplying it by a given orthogonal code. A scrambler 230 scrambles the output of the channel spreader 220 by multiplying it by a scrambling code. A second multiplexer 240 multiplexes the output of the scrambler 230 and the midamble signal as shown in FIG. 1. Here, the first multiplexer 210 and the second multiplexer 240 generate the frame structure of FIG. 1, under the control of a controller (not shown).
FIG. 3 illustrates a structure of a receiver in the conventional NB-TDD communication system. Referring to FIG. 3, a first demultiplexer 340 demultiplexes an input frame signal under the control of a controller (not shown), and outputs a midamble signal and other signals. Here, the “other signals” include the TFCI symbol, the data symbol, the SS symbol and the TCP symbol. A descrambler 330 descrambles the other signals output from the demultiplexer 340 by multiplying them by a scrambling code. A channel despreader 320 channel-despreads the output of the descrambler 330 by multiplying it by an orthogonal code. A second demultiplexer 310 demultiplexes the signals output from the channel despreader 320 into the TFCI symbol and other signals, under the control of the controller. Here, the “other signals” include the data symbol, the SS symbol, and the TCP symbol. A TFCI decoder 300 decodes the TFCI symbol output from the second demultiplexer 310 and outputs TFCI bits.
The TFCI is comprised of 1 to 2 bits to indicate 1 to 4 combinations of the services, comprised of 3 to 5 bits to indicate 8 to 32 combinations of the services, or comprised of 6 to 10 bits to indicate 64 to 1024 combinations of the services. Since the TFCI is information indispensable when the receiver analyzes the respective service frames, a transmission error of the TFCI may prevent the receiver from correctly analyzing the respective service frames. Therefore, the TFCI is encoded using an error correcting code so that even though a transmission error occurs on the TFCI, the receiver can correct the error.
FIG. 4 illustrates a scheme for encoding the TFCI using an error correcting code according to the prior art. Referring to FIG. 4, an extended Reed-Muller encoder 400 encodes an input 10-bit TFCI and outputs a 32-symbol TFCI codeword. A repeater 410 outputs intact even-numbered symbols of the TFCI codeword output from the extended Reed-Muller encoder 400 and repeats odd-numbered symbols, thereby outputting a total of 48 coded symbols. In FIG. 4, a less-than-10-bit TFCI is constructed to have a 10-bit format by padding a value of 0 from the MSB (Most Significant Bit), i.e., from the leftmost bit. The (32, 10) extended Reed-Muller encoder 400 is disclosed in detail in Korean patent application No. 1999-27932, the contents of which are hereby incorporated by reference.
In the (32, 10) extended Reed-Muller encoder 400, a minimum distance between codes is 12. After repetition, an input code is converted to a (48, 10) code having a minimum distance of 16. In general, an error correction capability of binary linear codes is determined depending on the minimum distance between the binary linear codes. The minimum distance (dmin) between the binary linear codes to become optimal codes is disclosed in a paper entitled “An Updated Table of Minimum-Distance Bounds for Binary Linear Codes” (A. E. Brouwer and Tom Verhoeff, IEEE Transactions on information Theory, VOL 39, NO. 2, MARCH 1993).
The paper discloses that the minimum distance required for the binary linear codes used to obtain a 48-bit output from a 10-bit input is 19 to 20. However, since the encoder 400 has a minimum distance of 16, the error correction encoding scheme of FIG. 4 does not have optimal codes, causing an increase in TFCI error probability in the same channel environment. Because of the TFCI error, the receiver may misjudge a rate of the data frame and decode the data frame at the misjudged rate, thereby increasing a frame error rate (FER). Therefore, it is important to minimize a frame error rate of the error correction encoder for encoding the TFCI.